This invention relates to a process for preparing synthetic crystalline zeolites having an effective pore sizes of about 4 and 10A in a form which is substantially useful in adsorbtion processes.
Microselective adsorbent of the zeolite type or crystalline aluminosilicates with a three-dimensional struction of silica, alumina, tetrahedral. This zeolite structure is characterized by a repeating three-dimensional network of large open aluminosilicate cages and are connected by small uniform openings and pores. Certain of these microselective adsorbents have been prepared synthetically from sodium silicate and sodium aluminate. After synthesis these large cavities are filled with water which can be driven off by heating without collapsing the cage. When dehydrated these cavities can readsorb large quantities of water or other vapor at low partial pressure. Due to the uniform structures or pore openings connecting the aluminosilicate cavities these zeolites exhibit the unique property of excluding larger molecules from the cavity and allowing small cavities to pass through and be adsorbed. Thereby acting as microselective adsorbents for molecules according their size and shape.
Because the zeolites are recovered from the manufacturing process as fine powders it is necessary to form the zeolites into nodular structures that can be packed into fixed beds for adsorption of gases or liquids from a stream. This is normally done by binding the zeolites with a material such as clay. The disadvantage of such a process however is that the clay is inactive and thus from 15 to 20% of the bed (depending on the percentage of clay used as a binder) is inactive.
U.S. Pat. No. 3,119,659 to Taggert discloses a process for preparing a so-called binderless sieve in which the clay is converted to zeolite by subsequent treatment to convert the clay binder to zeolite. This process involves a crystallization step in which the nodules are heated in a bath of sodium hydroxide solution to convert the clay binder to the zeolite.